Optical disk and optical disk apparatus

ABSTRACT

There is provided an optical disk which adopts a PRML (Partial Response and Maximum Likelihood) method for reproduction of recorded information and whose shortest pit has a conical shape without a bottom surface. If the PRML method is adopted, since it is not necessary to assure a large amplitude of a reproduction waveform of the shortest pit, the pit may have a conical shape. The conical pit enables recording with the dense shortest pit even in a conventional original disk recorder and original disk exposure process, thereby increasing a recording density in a dividing direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. application Ser. No.10/679,273, filed Oct. 7, 2003, and for which priority is claimed under35 U.S.C. §121. This application is based upon and claims the benefit ofpriority under 35 U.S.C. § 119 from the prior Japanese PatentApplication No. 2002-294008, filed Oct. 7, 2002, the entire contents ofboth applications are incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disk, and more particularlyto a shape of an information pit formed on an optical disk.

2. Description of the Related Art

In an optical disk, pits are carved on a transparent molded substrate ata part where data is recorded in advance, e.g., a pre-formatted portionof an RAM disk or an ROM disk. Such a pit is irradiated with a laserbeam through a molded substrate from a surface opposite to the surfaceon which the pit is formed, and information is read.

Although the pit is formed with different sizes in accordance withrecording information, its size is a sub-micron order. Forming a pit assmall and accurate as possible is important to increase a recordingdensity of the optical disk.

For example, in a current DVD-ROM , the shortest pit length (size in acircumferential direction) is 0.40 μm, a depth is approximately 100 nm,and a bottom forms a smooth conical trapezoid. In the conventional DVD,a time length of each pit is detected by slicing (binarizing) areproduction waveform by a predetermined threshold value, and reproducesinformation by converting this length into data. However, in order tocorrectly reproducing information by this method, the pit length must bestably formed, and a signal amplitude which is sufficient for enablingslicing must be obtained. Therefore, the shortest pit shape must beconical trapezoid with a flat bottom, and this is one factor determininga limit of the recording density. A bottom circumferential size of theshortest pit is stipulated to, e.g., (0.2 to 0.25)×(wavelength)/NA/1.14μm. Here, NA is a numerical aperture of an object lens.

As one conformation of an optical disk for coming generation, forexample, Jpn. Pat. Appln. KOKAI Publication No. 10-302310 proposes amode of reading a signal through a cover layer having a thickness ofapproximately 0.1 mm.

The optical disk having such a conformation is the same as aconventional optical disk in that a reflecting film is formed onirregularities provided to the molded substrate and this film isirradiated with a laser beam in order to read a signal. In case of thisconformation, however, as different from a conventional optical disksuch as a CD or a DVD, the laser beam is transmitted through a coverlayer instead of the molded substrate, and the reflecting film isirradiated with this laser beam.

The irregularities called the pit have a size of a sub-micron order, andhow correctly the small pit can be formed is one important element whichdetermines a signal quality and a recording density of the optical disk.

In the conventional optical disk which reads a signal by slicing areproduction waveform by using a predetermined threshold value andbinarizing it, the pit must be formed in such a manner that areproduction waveform amplitude can be stably and largely obtained evenin case of the shortest pit. Therefore, since a conical trapezoidalshape having a flat bottom surface must be assured even in the shortestpit, the recording density of the optical pit cannot be increased anyfurther.

Further, like the prior art, when the shortest pit shape is formed intoa conical trapezoid having a bottom surface and a large reproductionamplitude of a closest signal is assured, a difference between theshortest pit and the second shortest pit becomes small, and it is hardto discriminate a reproduction signal waveform of the shortest pit and areproduction signal waveform of the second shortest pit. This becomes aserious factor of erroneous reading of a signal.

Furthermore, in a cover layer type disk that a reflecting film surfaceis irradiated with a laser beam instead of a substrate through a coverlayer and information is read, or a Blu-Ray disc, when the shortest pitlength is dense to the limit in order to increase the recording density,forming a metallic reflecting film may fill up the shortest pit in somecases. In such a case, there occurs a problem that a reproduction signalis deteriorated.

It is, therefore, an object of the present invention to provide a pitshape which can readily increase an information recording density of anoptical disk.

BRIEF SUMMARY OF THE INVENTION

A shape of the shortest pit is determined as a conical shape having nobottom surface, and a PRML (Partial Response and Maximum Likelihood)mode is used for reproduction of recording information. When the PRMLmode is adopted, since it is not necessary to assure a large amplitudeof a reproduction waveform from the shortest pit, the pit may have aconical shape. When the conical pit can suffice, the shortest pit can bedense and information can be recorded even in a conventional originalboard recorder or original board exposure process, thereby increasing arecording density in a circumferential direction.

In a cover layer type disk, a bottom width in a molded substrate havingthe shortest pit is determined as 2×(reflecting film thickness)×sin (pitwall angle) (within ±20%). By doing so, the pit can be prevented frombeing filled up by formation of the reflecting film, while the pit has aconical shape with a size suitable for increasing the density afterformation of the film.

In this manner, the bit bottom width is stipulated, the conicaltrapezoidal pit is formed on the molded substrate, and the film isformed thereon. As a result, the bottom surface is filled, and theconical pit is formed. Consequently, the high-recording density pit withthe excellent asymmetry can be manufactured by using a current originalboard production process while suppressing irregularities in shape.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The file of this patent contains at least one photograph executed incolor. Copies of this patent with color photographs will be provided bythe Patent and Trademark Office upon request and payment of thenecessary fee.

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1A is a plane view showing a pit shape of a conventional opticaldisk, and FIG. 1B is a cross-sectional view;

FIG. 2A is a plane view showing a pit shape of an optical disk accordingto a first embodiment of the present invention, and FIG. 2B is across-sectional view;

FIG. 3 is a flow diagram of an optical disk manufacturing method;

FIGS. 4A and 4B show reproduction waveforms of respective pits;

FIG. 5 shows a reproduction waveform of an optical disk having a densityof 15 GB/plane class manufactured using a conventional pit shape;

FIG. 6 shows a reproduction waveform of an optical disk having a densityof 15 GB/plane class manufactured using a pit shape according to thepresent invention;

FIGS. 7A and 7B show pit shapes of an optical disk according to a secondembodiment of the present invention;

FIG. 8 is a cross-sectional view when a film is formed on the shortestpit of the optical disk according to the present invention; and

FIG. 9 is a block diagram showing a structure of an optical diskapparatus according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments according to the present invention will now be described indetail hereinafter with reference to the accompanying drawings. Thefollowing describes embodiments according to the present invention, anddoes not restrict an apparatus and a method according to the presentinvention.

FIG. 1A is a plane view showing a pit shape of a conventional opticaldisk; FIG. 1B, a cross-sectional view; FIG. 2A, a plane view showing apit shape of an optical disk according to a first embodiment of to thepresent invention; FIG. 2B, a cross-sectional view; FIG. 3, a flowdiagram of an optical disk manufacturing method; FIG. 4, a reproductionsignal waveform chart of each pit; FIG. 5, a reproduction waveform chartof an optical disk having a density of 15 GB/plane class manufacturedusing a conventional pit shape; and FIG. 6, a reproduction waveformchart of an optical disk having a density of 15 GB/plane classmanufactured using a pit shape according to the present invention.

In the optical disk, usually, the disk on which pits such as shown inFIG. 1 are recorded is read by using a laser beam, and the informationis reproduced. The laser beam enters the optical disk in a directionindicated by an arrow in FIG. 1B, and the information is read based onan intensity of a reflected light beam.

In this embodiment, although it is determined the disk has a diameter of120 mm and a thickness of 1.2 mm (lamination of two substrates eachhaving a thickness of 0.6 mm) and it is an ROM disk dedicated toreproduction, the present invention is not restricted thereto, it ispossible to adopt a disk having a cover layer of 0.1 mm attached to asubstrate of 1.1 mm, and it includes a pre-pit portion of an RAM disk.

In FIG. 1, reference numeral 10 denotes the shortest pit; 11, anotherpit; 12, a depth of the shortest pit; and 13, a depth of another pit. InFIG. 2, reference numeral 20 designates the shortest pit; 21, anotherpit; 22, a depth of the shortest pit; and 23, a depth of another pit.

A method of manufacturing a disk having such pits will now be describedhereinafter with reference to FIG. 3. First, as an original board, aglass original board 31 having a surface polished and cleansed is used(ST 1). A photoresist 32 is applied to the glass original board surface(ST 2), and this surface is exposed by using a laser beam, therebyrecording information (ST 3). Then, the photoresist on the exposed glassoriginal board is etched, irregularities of pits are formed (ST 4). Thisglass original board is subjected to plating processing, therebyproducing a stamper 33 (ST 5). This stamper 33 is used as a die, and aresin (generally, polycarbonate) molded plate 34 is manufactured byinjection molding (ST 6). Thereafter, a reflecting film or a recordingfilm is formed on the molded plate 34 (ST 7), and this plate is attachedto another molded plate 35 manufactured in the similar manner (ST 8),thereby bringing the optical disk to completion. In this embodiment, athickness of each of the molded substrates 34 and 35 is 0.6 mm.

In the conventional optical disk, a pit bottom surface is formed flateven in case of the shortest pit as shown in FIG. 1. That is because anamplitude of a reproduction signal from the shortest pit tends to becomevery small and binarization is impossible by slicing a reproductionwaveform by a predetermined threshold value as it is. Using a conicaltrapezoid in this manner can increase the amplitude slightly and enableslicing. In this case, only a pit length of the shortest pit is setlonger than a theoretical value. For example, when a pit correspondingto a 2 T code (T: a length corresponding to a reference clock cycle) isthe shortest pit, a pit length of the 2 T code is set longer than ⅔ of apit length of, e.g., a 3 T code. That is, only the pit of the 2 T codehas a length which is not proportionate to a code value.

When manufacturing a disk based on steps such as shown in FIG. 3, adepth of the shortest pit depends on a film thickness of the photoresist32, a tilt angle of a wall surface of the pit depends on an intensitydistribution in a laser spot and a characteristic and an etchingcondition of the photoresist 32. A depth of the pit is restricted toλ/4n (λ: a reproduction wavelength, n: a refractive index of asubstrate) in case of an optically ROM in order to obtain a reproductionsignal, and a tilt angle of the pit is approximately 40 degrees in thecurrent process technology. In this case, in order to flatten the bottomsurface of the shortest pit as shown in FIG. 1, the length of theshortest pit cannot be greatly reduced. Therefore, it can be understoodthat the recording density in the disk circumferential direction is notvery high.

On the other hand, in the optical disk for coming generation, it hasbeen examined that information is reproduced by using a mode called aPRML (Partial Response and Maximum Likelihood) method in place ofreading a signal by a binarization method based on slicing of areproduction waveform like the prior art. This mode converts areproduction signal from each pit into multiple values based on itswaveform and amplitude level. In case of this mode, it is desirable thatthe reproduction signal from the shortest pit is small so that it can berecognized as a signal from the shortest pit. Therefore, it is notnecessary to form a pit bottom surface by setting only the pit of theshortest code longer than a value which is proportionate to the code andincrease the signal amplitude as described above. On the contrary, sincethe PRML reads information on the amplitude level, centers of thereproduction signal amplitudes from the respective pits must match witheach other. That is, the PRML cannot be applied unless the asymmetry(which will be described later) is close to 0.

Base on these points, the pit shape which realizes a high-recordingdensity suitable for the PRML is the first embodiment according to thepresent invention shown in FIG. 2. A characteristic of this pit shapelies in that the shortest pit does not have a flat bottom surface buthas a conical shape and its depth 22 is smaller than a depth 23 ofanother pit. An inclination of a wall surface of the shortest pit isequal to that of the conventional pit shown in FIG. 1.

The conventional shortest pit shape which is a conical trapezoidal shapehas a limit in reducing the shortest pit length since the tilt angle ofthe pit wall surface is gentle. However, if this conical shape isadopted, even the current original board process technology can greatlyreduce the shortest pit length, thereby considerably increasing therecording density.

Since the shortest pit has a conical shape and its bottom surface is notflat, a reproduction amplitude from the shortest pit becomes small.Further, irregularities in pit size (mainly a depth 22) caused due tounstableness of the exposure process slightly become large. However,these phenomena which are serious drawbacks in reproduction by the slicemethod hardly become shortages when using the PRML method.Conventionally, there is no optical disk product having a conical pitnor a concept of reading the conical pit by using the PRML method.

In this embodiment, since the bottom surface of the shortest pit is notformed, the asymmetry of the reproduction waveform can approximate 0.FIG. 4A shows a reproduction signal waveform obtained when reproducing apit having a conventional shape, and FIG. 4B shows a reproduction signalwaveform obtained when reproducing a pit having a shape according to thepresent invention. In FIGS. 4, reference character W2T denotes areproduction signal waveform of a 2 T code pit (shortest pit); W3T, areproduction signal waveform of a 3 T code pit; WMT, a reproductionsignal waveform of a longest code pit; A2T, an amplitude of a 2 T codepit reproduction signal; AMT, an amplitude of a longest code pitreproduction signal; L2T, a central level of a 2 T code pit reproductionsignal waveform; LMT, a central level of a reproduction signal waveformof a longest code pit; and D, a level difference (LMT−L2T) obtained bysubtracting the central level L2T from the central level LMT.

Although the asymmetry can be defined with respect to the pitreproduction signal of each code, it is determined as the asymmetry ofthe 2 T code which is most important in the present invention. That is,here, the asymmetry is determined as a value (D/AMT) obtained bydividing the level difference D by the reproduction signal waveformamplitude AMT of the longest code pit.

Assuming that an area where the pit is formed is a pit area and an areawhere no pit is formed is a mirror area, the mirror area has a highreflected light level than that of the pit area (see FIGS. 1 and 2). Inthe prior art as shown in FIG. 4A, although the asymmetry has a largevalue, this is because the pit length of the conventional shortest pitis set longer than a value which is in proportion to the code and thecentral level of the reproduction signal waveform of the shortest pit islower than that of a pit of any other code.

As shown in FIG. 4B, in regard to the reproduction signal waveform ofthe pit having the shape according to the present invention, thereproduction signal amplitude A2T of the shortest pit is smaller thanthat of the prior art shown in FIG. 4A, and the asymmetry (D/AMT) alsobecomes small. That is because the shortest pit has a conical shape andthe reflected light level in the vicinity of the area where the shortestpit is formed is higher than that of the prior art.

Furthermore, since the signal amplitude of the shortest pit is small inaccordance with the pit length, it is easy to discriminate the shortestpit (e.g., 2 T) and the second shortest pit (e.g., 3 T). Thus, conicalshape of the shortest pit is advantageous in PRML reproduction.According to the pit shape of the present invention, the asymmetry canreadily approximate +0.10 or lower (preferably within a range of ±10).Defining a value obtained by dividing the shortest pit reproductionsignal amplitude A2T by the maximum amplitude AMT (A2T/AMT) as aresolution, a resolution of not more than 15% can be easily realized inthe present invention. It is to be noted that, in the present invention,the 2 T code pit is preferably constantly formed into a conical shape inorder to facilitate identification of the 2 T code pit (shortest pit)and the 3 T code pit. That is, when the shortest pit (2 T) is formed inproportion to the code (3 T, 4 T, . . .), the shortest pit is alwaysformed into a conical shape even if the shortest pit has a flat bottomportion. In such a case, a circumferential length of the shortest pit (2T) is not in proportion to the pit lengths of other codes.

FIGS. 5 and 6 respectively show results of manufacturing disks of 15GB/plane class with the pit shape according to the prior art and the pitshape according to the present invention by trial and reproducinginformation.

FIG. 5 shows a signal reproduction waveform of an optical disk havingthe conventional pit shape (FIG. 1), and the jitter obtained by theslice method is 12.6% since the signal amplitude A2T of the shortest pit(2 T) is large, but reproduction by the PRML method cannot be performedbecause of the large asymmetry and an error rate cannot be measured. Onthe other hand, FIG. 6 shows a pit shape according to the presentinvention (FIG. 2), and the jitter is 13.4% which is relatively badsince the shortest signal amplitude A2T is small, but the asymmetry isgood and an error rate is 4×10−6 in reproduction by PRML, which is alevel making practicable.

As described above, in the optical disk that information is reproducedby the PRML method, using the pit shape according to the presentinvention can increase the recording density in a disk tangentialdirection even in the conventional original board process, and theasymmetry is also improved, thereby enhancing the error rate.

A second embodiment according to the present invention will now bedescribed. FIGS. 7 are plane views showing a pit shape of an opticaldisk according to the present invention, and FIG. 8 is a cross-sectionalview when forming a film on the shortest pit of the optical pitaccording to the second embodiment of the present invention.

In FIG. 7A, reference numeral 50 denotes the shortest pit, and 51designates another pit. In FIG. 7B, reference numeral 52 denotes amolded substrate; 53, a reflecting film; x, a shortest bit bottom widthon the molded substrate. In the present invention, a target is anoptical disk which is of a type that a laser beam enters from a sideopposite to the molded substrate 52 as indicated by an arrow in FIG. 7B(e.g., a disk which is of a surface recording type or a type thatinformation is read through a cover layer of approximately 0.1 mm or anL1 layer in a conventional two-layer DVD disk). That is, an incidentdirection of the laser beam is an opposite direction as compared withthe optical disk shown in FIG. 1 or 2.

In the conventional optical disk, since the recording density is low,the bottom width x of the shortest pit is sufficiently wider than a filmthickness of the reflecting film 53. Therefore, there occurs no problemeven if the reflecting film is formed on the bit, the film is irradiatedwith the light from the direction indicated by an arrow and a signal isread. However, in case of the high-density optical disk for cominggeneration, when using the PRML mode in signal reproduction or whenusing a code series that the shortest pit is 2 T in particular, sincethe recording line density is very high, the bottom width x of theshortest pit approximates 0 in the current original board manufacturingprocess technology. In this case, the shortest pit is filled by formingthe film, and reading the signal from the direction indicated by anarrow deteriorates the reproduction signal.

On the contrary, when the bottom width of the shortest pit is set largerthan a value which is in proportion to the code, this becomesdisadvantageous for increasing the density, and discrimination of theshortest pit and the second shortest pit becomes difficult because ofthe extremely large signal from the shortest pit, thereby degrading theasymmetry. In particular, since the asymmetry is important when usingthe PRML mode in signal reproduction, the pit length of the shortest pitcannot be set larger than a value which is in proportion to the code inthe light of this point.

For the reasons mentioned above, the shortest pit must be set as smallas possible in a range which does not fill the shortest pit and does notlead to degradation in the signal. Thus, in the present invention, asshown in FIG. 8, the bottom width x of the shortest pit is determined asx=2×d×sinθ (within ±20%) (d: a thickness of the reflecting film, 0: awall angle of the pit).

That is, in the optical disk according to this embodiment, thecross-sectional shape of the shortest pit is trapezoidal in the moldedsubstrate and triangular on the surface of the reflecting film. Namely,when manufacturing such an optical disk, as to the shortest pit, the pitcross-sectional shape before forming the reflecting film is trapezoidal,and the pit cross-sectional shape after forming the reflecting film istriangular.

By stipulating the shortest pit to this size, the pit is not filled byformation of the film, and the signal is not deteriorated. Additionally,since the conical pit can be obtained after forming the film, theintensity of the signal from the shortest pit can be appropriatelysuppressed. As a result, it is possible to prevent deterioration in theasymmetry or erroneous reading of signals from pits with any othersizes, especially erroneous reading of the 2 T and 3 T pits.

In the optical disk manufactured by the current original boardmanufacturing process technology, the wall angle θ of the pit isgenerally approximately 40 degrees. Further, if a blue laser beam isused as a reproduction light ray, when trying to obtain the reflectivityof, e.g., approximately 70%, the reflecting film thickness ofapproximately 25 nm is required in case of Al (aluminium), andapproximately 50 nm is required in case of Ag (silver). Applying theseparameters to the expression of this embodiment, the bottom width of theshortest pit is 32±6 nm in case of Al, and it is 64±13 nm in case of Ag.

It is to be noted that only the bottom width x of the shortest pit isnot in proportion to the bottom width of another pit. That is, in caseswhere the shortest pit is the 2 T code pit, the bottom width x is not ⅔of, e.g., the 3 T code pit, but it is set in accordance with a thicknessof the reflecting film. Furthermore, the bottom width x is a value whichis not directly proportionate to the recording density.

Advantages obtained when stipulating the shortest pit to this size willnow be described. First, when manufacturing the disk, the pit shape andthe reproduction signal are stabilized since the pit cross section ofthe molded substrate has a trapezoidal shape. That is because, when thecross section of the molded substrate has a triangular shape,irregularities in original board exposure condition or irregularities infilm thickness when forming the film scatter the pit size, and thereproduction signal from that pit is sensitively affected by the pitsize and becomes unstable. As another advantage, this size is a limitsize that the pit is not filled when forming the film. As a result, thepit can become dense to the limit density which does not fill the pitand does not deteriorate the reproduction signal. Moreover, since thebottom is filled and the conical pit shape can be obtained after formingthe film, the reproduction signal from the shortest bit does not becometoo large, and the asymmetry approximates 0, which is also advantageousin discrimination of the signal.

As described above, in the optical disk to which the light enters fromthe surface opposite to the molded substrate in order to reproduceinformation, the shortest pit has a conical shape after forming the filmby using the pit shape according to the present invention, and thesignal waveform from the shortest pit can be appropriately obtainedwhile increasing the recording density.

Description will now be given as to an embodiment of an optical diskapparatus which records/reproduces information by using the optical diskhaving the pit with the above-described shape formed thereto. FIG. 9 isa block diagram showing a structure of the optical disk apparatusaccording to this embodiment.

An optical disk 61 is an optical disk dedicated to reading or an opticaldisk which can record user data. The disk 61 is rotated and driven by aspindle motor 63. Recording and reproduction of information with respectto the optical disk 61 are carried out by an optical pickup head (whichwill be referred to as a PUH hereinafter) 65. The PUH 65 is connected toa thread motor 66 through a gear, and this thread motor 66 is controlledby a thread motor control circuit 68.

A seek destination address of the PUH 65 is inputted to the thread motorcontrol circuit 68 from a CPU 90, and the thread motor control circuit68 controls the thread motor 66 based on this address. A permanentmagnet is fixed inside the thread motor 66, and the PUH 65 moves in aradial direction of the optical disk 61 when a drive coil 67 is excitedby the thread motor control circuit 68.

To the PUH 65 is provided an object lens 70 which is supported by a wireor a flat spring which is not illustrated. The object lens 70 can movein a focusing direction (direction of an optical axis of the lens) bydrive of a drive coil 72, and it can move in a tracking direction(direction orthogonal to the optical axis of the lens) by drive of adrive coil 71.

A semiconductor laser 79 emits a laser beam by a laser drive circuit 75in a laser control circuit 73. The optical disk 61 is irradiated withthe laser beam emitted from the semiconductor laser 79 through acollimator lens 80, a half prism 81 and an object lens 70. The reflectedlight from the optical disk 61 is led to a photodetector 84 through theobject lens 70, the half prism 81, a condensing lens 82 and acylindrical lens 83.

The photodetector 84 consists of, e.g., four divided photodetectorcells, and a detection signal from each divided photodetector cell isoutputted to an RF amplifier 85. The RF amplifier 85 combines signalsfrom the photodetector cells, and generates a focus error signal FEindicative of an error from just focusing, a tracking error signal TEindicative of an error between a beam spot center of the laser beam anda track center and an RF signal which is a full addition signal of thephotodetector cell signals.

The focus error signal FE is supplied to the focusing control circuit87. The focusing control circuit 87 generates a focus control signal FCin accordance with the focus error signal FE. The focus control signalFC is supplied to the drive coil 72 in the focusing direction, and focusservo is carried out so that the laser beam is constantly just focusedon the recording film of the optical disk 61.

The tracking error signal TE is supplied to the tracking control circuit88. The tracking control circuit 88 generates a tracking control signalTC in accordance with the tracking error signal TE. The tracking controlsignal TC is supplied to the drive coil 72 in the tracking direction,and tracking servo is carried out so that the laser beam constantlytraces on the track formed on the optical disk 61.

When the focus servo and the tracking servo are effected, a change inreflected light from, e.g., the pit formed on the track of the opticaldisk 61 is reflected to the full addition signal RF of output signalsfrom the respective photodetector cells of the photodetector 84. Thissignal is supplied to a data reproduction circuit 78. The datareproduction circuit 78 reproduces recorded data based on a reproductionclock signal from a PLL circuit 76.

When the object lens 70 is controlled by the tracking control circuit88, the thread motor 66, i.e., the PUH 65 is controlled by the threadmotor control circuit 68 in such a manner that the object lens 70 ispositioned in the vicinity of a predetermined position in the PUH 65.

The motor control circuit 64, the thread motor control circuit 68, thelaser control circuit 73, the PLL circuit 76, the data reproductioncircuit 78, the focusing control circuit 87, the tracking controlcircuit 88, an error correction circuit 62 and others are controlled bya CPU 90 through a bus 89. The CPU 90 comprehensively controls thisrecording/reproducing apparatus in accordance with an operation commandprovided from a host device 94 through an interface circuit 94.Furthermore, the CPU 90 uses an RAM 91 as a working area and performs apredetermined operation in accordance with a program recorded in an ROM92.

The data reproduction circuit 78 reproduces information by abinarization method which binarizes an information reproduction signalwaveform by slicing using a threshold voltage or a PRML method whichconverts an amplitude of the information reproduction signal waveforminto multiple values. It is designed to reproduce information of theoptical disk that the asymmetry of a reproduction signal is not morethan +0.10 and a ratio of a signal amplitude of the shortest pitrelative to a signal amplitude of the longest pit, i.e., the resolution(A2T/AMT) is not more than 15% when reproducing information by the PRMLmethod.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventionconcept as defined by the appended claims and their equivalents.

1. (canceled)
 2. An optical disk comprising a reflecting film formed ona molded substrate having pits indicative of information, theinformation being read from the reflecting film side by using a laserbeam, a cross-sectional shape of the shortest pit is trapezoidal in themolded substrate, and it is triangular on a surface of the reflectingfilm.
 3. The optical disk according to claim 2, wherein a bottom width xof the shortest pit cross section in the molded substrate is as follows:x=2·d·sinθ (within±20%) wherein θ is a tilt angle of a wall surface ofthe shortest pit in the molded substrate, and d is a film thickness ofthe reflecting film.
 4. The optical disk according to claim 2, wherein apit cross section of a pit other than the shortest pit has a trapezoidalshape in both the molded substrate and the reflecting film surface. 5.The optical disk according to claim 3, wherein a pit cross section of apit other than the shortest pit has a trapezoidal shape in both themolded substrate and the reflecting film surface.
 6. A method ofmanufacturing an optical disk by forming a reflecting film on a moldedsubstrate having pits indicative of information formed thereto, theshortest pit cross-sectional shape before forming the reflecting film istrapezoidal and the shortest pit cross-sectional shape after forming thereflecting film is triangular.